Rubber composition for tire tread

ABSTRACT

A rubber composition for a tire tread which is provided by the present invention can be obtained by blending 100 parts by weight of a natural rubber and/or a diene synthetic rubber with 
     40 to 100 parts by weight of a carbon black having characteristic values of a cetyltriammonium bromide adsorption specific surface area (CTAB) being 90 to 220 m 2  /g and a 24M4DBP absorption value being 90 to 140 ml/100 g, and 
     0.1 to 20 parts by weight of an organic unsaturated fatty acid having two or more carbon double bonds in its molecule which includes 10% by weight or more of a conjugated diene acid having at least one pair of two carbon double bonds in a conjugated relation in its molecule. 
     This rubber composition for a tire tread can be suitably used for a pneumatic tire, particularly a pneumatic tire for heavy load having an excellent wear resistance.

TECHNICAL FIELD

The present invention relates to a rubber composition for a tire tread,and particularly, it relates to a rubber composition for a tire treadsuitable for a pneumatic tire for heavy load having excellent wearresistance.

BACKGROUND ART

Heretofore, as a filler for a rubber having a high reinforcing effect, acarbon black of an ISAF class is well known. However, in the pneumatictire for heavy load in recent years, the carbon black which can improvewear resistance has been desired in accordance with demands of improvedcase durability, resource saving and good mileage.

Thus, the carbon black which can be used as the filler in the rubbercomposition for the tread of the tire for heavy load has also changedfrom the ISAF class to an SAF class.

By the way, in order to improve the wear resistance of rubber productssuch as the tires, it is already known that a high structure carbonblack having a fine particle diameter is suitable.

The diameter of the fine particles of the carbon black can be determinedby measuring the particles in an electron micrograph, but the thusmeasured particle diameter is an average value and in fact, it has acertain distribution.

Alternatively, the particle diameter can also be calculated by measuringthe surface area of the carbon black, and for this measurement, an N₂ SAmethod for determining from the amount of an adsorbed nitrogen gas andan iodine adsorption method (an IA method) are simple. These methodshave been routinely employed in a quality control of a maker, anacceptance test of a user, and the like.

In measuring the surface area, a point to which much attention should bepaid is whether a value obtained by the measurement principle denotes awhole surface area or an external surface area.

It is considered that small pores are present on the surfaces of thecarbon black, and the whole surface area also includes the surface areain the pores, but the external surface area does not include the surfacearea in the pores. When the carbon black is kneaded as a reinforcingagent with a rubber, such giant molecules as in the rubber cannot getinto the pores, and for this reason, the surface of the pores cannot beeffectively utilized in practice.

Therefore, the external surface area of the carbon black is also calleda rubber effective surface area sometimes. In order to determine thisrubber effective surface area, there is used a surface area calculatedfrom an amount of adsorbed cetyltriammonium bromide (CTAB) having alarge molecular weight. This is called a CTAB method surface area. ThisCTAB method surface area has a unit of m² /g, and it can be measured inaccordance with ASTM D-3765-89.

On the other hand, the particles of the carbon black are not presentsingly but integrally like bunches of grapes, as seen from an electronmicroscope. This is called a structure. This structure can be classifiedinto two kinds, and one is established by the mutual fusion of theparticles, i.e., a chemical bond and the other is established by aphysical bond such as a van der Waal force. The former non-destructivestructure morphology is called a primary structure, and the latterdeformable and destructive structure morphology is called a secondarystructure. The structure can usually be evaluated by an absorption valueof dibutyl phthalate (DBP). This principle is based on a phenomenon thatthe carbon black having high entanglement properties among the particlescan absorb a larger amount of an oil.

A 24M4DBP absorption value is a value obtained by repeating acompression operation at a pressure of 24,000 psi four times and thenmeasuring a DBP absorption value in accordance with ASTM D-3493, andthis absorption value is an index for evaluating the skeleton structureproperties of the carbon black mainly comprising the primary structure,used in measuring the DBP absorption value, on the basis of thenon-destructive real structure morphology (the primary structure) to theexclusion of the DBP absorption value by the deformable and destructivestructure morphology (the secondary structure) formed by the so-calledvan der Waal force.

In order to improve the wear resistance by adding the carbon black tothe rubber, it is required that the carbon black is brought into fineparticles. However, if the carbon black is pulverilzed more finely thanin SAF to form a higher structure, the dispersibility of the carbonblack deteriorates noticeably in the blended rubber. Therefore, such animprovement of the wear resistance as to be expected cannot be observed,and nowadays, the improvement of the wear resistance has already reacheda substantial limit. Furthermore, if the carbon black is pulverized morefinely than in SAF to form a higher structure, the viscosity of theunvulcanized rubber becomes very high, so that workability deterioratesnoticeably.

On the other hand, as a method for improving the wear resistance withoutusing the carbon black, there is a means using cis-1,4-polybutadienerubber (hereinafter referred to simply as "high-cis BR"). In order toimprove the wear resistance by the use of the high-cis BR, means such asthe increase of a molecular weight, the betterment of the linearity of apolymer and the reduction of a molecular weight distribution have beenheretofore taken. However, as in the case of the formation of the carbonblack having the fine particle size, the dispersibility of the carbonblack deteriorates noticeably in the blended rubber. Therefore, such animprovement of the wear resistance as to be expected is not observed,and nowadays, the improvement of the wear resistance has reached asubstantial limit. Furthermore, the viscosity of the unvulcanized rubberbecomes very high, so that workability deteriorates noticeably.

For the purpose of improving the dispersibility of the carbon black andinhibiting the viscosity of the unvulcanized rubber from increasing,there have been heretofore added various kinds of softening agents andplasticizers such as cotton seed oil, soybean oil, pine oil, aroma oil,nathphenic oil and dioctyl phthalate. The addition of the softeningagent and the plasticizer can improve the dispersibility and inhibit theincrease of the viscosity, but the wear resistance deterioratesconversely.

Furthermore, it has also be attempted to utilize a reactive softeningagent and plasticizer with the intention of improving the dispersibilityand inhibiting the increase of the viscosity. In this case, the wearresistance can be improved as compared with the employment of theabove-mentioned softening agent and plasticizer, but the degree of itsimprovement is not sufficient and it is the most to stem thedeterioration of the wear resistance.

We have intensively investigated in view of this point, and we havesuggested a rubber composition having improved static mechanicalstrength obtained by blending, as a vulcanizing accelerator, a specificunsaturated fatty acid, i.e., an organic unsaturated fatty acidincluding two or more carbon double bonds in the molecule which includes5% by weight or more of a conjugated diene acid having at least oneconjugated carbon-carbon double bond in its molecule (Japanese PatentApplication Laid-open No. 189850/1992).

However, when the specific unsaturated fatty acid is merely added, theimprovement effect of dynamic mechanical strength, particularly the wearresistance is poor, though the static mechanical strength can beimproved. Thus, a further beneficial suggestion has been desired.

DISCLOSURE OF THE INVENTION SUMMARY OF THE INVENTION

The present invention has been developed to solve the above-mentionedproblems of conventional techniques, and an object of the presentinvention is to provide a rubber composition for a tire treadparticularly in a pneumatic tire for heavy load which is remarkablyimproved in wear resistance.

The present inventors have paid much attention to a conception that anorganic unsaturated fatty acid having a conjugated diene acid which iseffective to improve static destructive properties as a vulcanizingaccelerator for a rubber composition of car tire itself would beeffective to prevent the deterioration of carbon black dispersibilityand the increase of viscosity which disturbs the improvement of wearresistance, and they have intensively investigated. As a result, it hasbeen found that when a specific carbon black is restrictively used in acombination of a natural rubber and a diene synthetic rubber and whenthe above-mentioned organic unsaturated fatty acid having the conjugateddiene acid is blended, a rubber composition for a tire tread which ismore excellent in wear resistance than in the conventional cases can beobtained. As a result of further researches, the present invention hasbeen completed.

That is to say, a rubber composition for a tire tread of the presentinvention is obtained by blending 100 parts by weight of a naturalrubber and/or a diene synthetic rubber with

40 to 100 parts by weight of a carbon black having characteristic valuesof a cetyltriammonium bromide adsorption specific surface area (CTAB)being 90 to 220 m² /g and a 24M4DBP absorption value being 90 to 140ml/100 g and

0.1 to 20 parts by weight of an organic unsaturated fatty acid havingtwo or more carbon double bonds in the molecule which includes 10% byweight or more of a conjugated diene acid having at least one conjugatedcarbon-carbon double bond in its molecule.

The carbon black preferably has a cetyltriammonium bromide adsorptionspecific surface area (CTAB) of 120 to 220 m² /g and a 24M4DBPPabsorption value of 100 to 140 ml/100 g.

The diene synthetic rubber is preferably one selected from the groupconsisting of a polybutadiene, a styrene•butadiene copolymer rubber, asynthetic polyisoprene rubber, a chloroprene rubber, a butyl rubber andan ethylene•propylene terpolymer rubber.

The content of the conjugated diene acid in the organic unsaturatedfatty acid is preferably 25% by weight or more.

The natural rubber and/or the high-cis BR is preferably contained in anamount of 70% by weight or more in 100 parts by weight of the totalrubber components.

The high-cis BR preferably has characteristic values of the following(a):

(a) an Mw is 400,000 or more and an Mw/Mn is less than 4.0 wherein theMw is a weight-average molecular weight and an Mn is a number-averagemolecular weight.

A mixing ratio of the natural rubber and the high-cis BR is preferablysuch that the amount of the natural rubber is 70% by weight or more.

Detailed Description of the Invention

A rubber which can be used in the present invention is fundamentally anatural rubber and/or a diene synthetic rubber. Examples of the usablediene synthetic rubber include a polybutadiene rubber (BR), astyrene•butadiene copolymer rubber (SBR), a synthetic polyisoprenerubber, a chloroprene rubber (CR), a butyl rubber (IIR) and anethylene•propylene•diene three-dimensional copolymer rubber (EPDM).

Among the diene synthetic rubbers, a high-cis BR is particularlypreferable, and in the case of a combination of the natural rubber (NR)and the high-cis BR, a ratio of the natural rubber (NR) to the high-cisBR is 80/20 to 30/70, preferably 70/30 to 40/60. In addition, the amountof the combination of the natural rubber and the high-cis BR is 70% byweight or more, preferably 80% by weight or more, more preferably 85% byweight or more. If the amount of the high-cis BR is less than theabove-mentioned range, the effect of wear resistance is poor.

As the rubber component for use in the natural rubber and the high-cisBR, at least one rubber which can be selected from the group consistingof the above-mentioned SBR, BR, CR, IIR and EPDM can be used in anamount of 30% by weight, preferably 20% by weight, more preferably 15%by weight.

In the high-cis BR, it is required that an Mw is 400,000 or more and anMw/Mn is less than 4.0, wherein the Mw is a weight-average molecularweight and an Mn is a number-average molecular weight. If the Mw is lessthan 400,000 and the Mw/Mn is 4.0 or more, the wear resistancedeteriorates unpreferably.

The Mw is in the range of 400,000 to 2,000,000, preferably 500,000 to1,500,000, more preferably 500,000 to 1,000,000, and the Mw/Mn is lessthan 4.0, preferably less than 3.5.

Furthermore, in the high-cis BR which meets the above-mentioned Mw andMw/Mn, an ML₁₊₄ (100° C.) is 38 or more, preferably in the range of 40to 120. If the ML₁₊₄ (100° C.) is less than 38, the wear resistancedeteriorates unpreferably.

Moreover, a bonded cis content of the high-cis BR is preferably 90% ormore, more preferably 93% or more.

A carbon black which can be used in the present invention is required tohave characteristic values, i.e., a CTAB method surface area of 90 to220 m² /g, preferably 120 to 220 m² /g, and a 24M4DBP absorption valueof 90 to 140 ml/100 g, preferably 100 to 135 ml/100 g.

If the CTAB and the 24M4DBP of the carbon black are outside the range of90 to 220 m² /g and the range of 90 to 140 ml/100 g, respectively, thewear resistance effect of the present invention cannot be exertedsufficiently.

The rubber composition of the present invention is required to containthe carbon black having the above-mentioned characteristic values in anamount of 40 to 100 parts by weight, preferably 45 to 80 parts byweight, more preferably 48 to 70 parts by weight. If the amount of thecarbon black is less than 40 parts by weight or more than 100 parts byweight, the wear resistance effect of the present invention cannot beexerted sufficiently.

In the organic unsaturated fatty acid which can be used in the presentinvention, "a conjugated diene acid" means an organic unsaturatedmonocarboxylic acid having at least one conjugated carbon-carbon doublebond in its molecule, and the conjugated diene acid preferably has onepair of the carbon double bonds in the conjugated relation, but it mayhave two or more pairs of the carbon double bonds in the conjugatedrelation.

The organic unsaturated fatty acid containing 10% by weight or more ofthe conjugated diene acid and having two or more carbon double bonds inthe molecule (hereinafter referred to simply as "organic unsaturatedfatty acid"), needless to say, contains the conjugated diene acid, butthe other organic unsaturated fatty acids each contain two or morecarbon double bonds, which are not in the mutually conjugated relation.

The content of the conjugated diene acid in the organic unsaturatedfatty acid is 10% by weight or more, preferably 25% by weight or more,more preferably 35% by weight or more.

If the content of the conjugated diene acid is less than 10% by weight,the wear resistance effect of the present invention cannot be exertedsufficiently. Conversely, if the content of the conjugated diene acid is25% by weight or more, the wear resistance effect can be exerted moreeffectively.

Examples of the conjugated diene acid include 2,4-pentadienoic acid,2,4-hexadienoic acid, 2,4-decadienoic acid, 2,4-dodecadienoic acid,9,11-octadecadienoic acid, α-eriostearic acid,9,11,13,15-octadecatetraenoic acid and 9,11,13-octadecatrienoic acid.

A preferable example of the organic unsaturated fatty acid containingthe conjugated diene acid is a dehydrated castor oil fatty acid obtainedby subjecting castor oil to a dehydration reaction. Furthermore, thecontent of the conjugated diene acid can be changed by selecting adehydration manner, and for example, the organic unsaturated fatty acidcontaining 35% by weight or 60% by weight of the conjugated diene acidcan be obtained.

In this dehydrated castor oil fatty acid, 9,11-octadecadienoic acid ismainly contained as the conjugated diene acid. In the other organicunsaturated fatty acid, non-conjugated octadecadienoic acid is mainlycontained, and additionally, linoleic acid or linolenic acid can also becontained. In the present invention, in addition to the dehydratedcastor oil fatty acid, a fatty acid typified by stearic acid which hasbeen heretofore used may be used together.

The organic unsaturated fatty acid which can be used in the presentinvention preferably contains 75% by weight or more of an unsaturatedfatty acid having a long-chain alkyl group having 10 to 22 carbon atoms,preferably 12 to 20 carbon atoms in the total unsaturated fatty acid. Ifthe unsaturated fatty acid having 10 to 22 carbon atoms is used, a highmodulus of elasticity can be obtained.

Furthermore, it is preferred that an unsaturation degree of the totalunsaturated fatty acid is in the range of 130 to 180 in terms of aniodine value. If the iodine value is in the range of 130 to 180, thehigh modulus of elasticity can be obtained.

In the rubber composition of the present invention, the above-mentionedorganic unsaturated fatty acid is required to be contained in the rangeof 0.1 to 20 parts by weight, preferably 0.3 to 10 parts by weight, morepreferably 0.5 to 4 parts by weight. If the content of the organicunsaturated fatty acid is less than 0.1 part by weight or more than 20parts by weight, the wear resistance effect of the present inventioncannot be exerted sufficiently.

The rubber composition for the tire tread of the present invention canbe blended with a softening agent in addition to the organic unsaturatedfatty acid, and the amount of the softening agent to be blended is 10parts by weight or less, preferably 6 parts by weight or less.

In the present invention, additives can be suitably used in addition tothe above-mentioned materials to be blended, and these additives arethose which can usually be used as additives for the rubber. Examples ofthe additives include a vulcanizing agent, a vulcanizing accelerator, avulcanizing acceleration auxiliary, an antioxidant and a foaming agent.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES

Next, the present invention will be described in more detail withreference to examples and comparative examples, but the scope of thepresent invention should not be limited to these examples.

(1) Preparation of a rubber composition for a tire tread

Examples 1 to 3 and Comparative Examples 1 to 14

In accordance with blend ratios in Tables 1 to 3, materials other thansulfur and a vulcanizing accelerator were mixed with a raw materialrubber (a natural rubber) in a Banbury type internal mixer, and sulfurand the vulcanizing accelerator were then added to the resultant masterbatch in an open roll mill to prepare rubber compositions for tiretreads. Tables 1 to 3 show the measured results of tests, i.e., wearresistance, workability and carbon dispersibility of the respectiverubber compositions.

Examples 4 to 10 and Comparative Examples 15 to 36

In accordance with blend ratios (blend unit: parts by weight) in Tables4 to 8, materials other than sulfur and a vulcanizing accelerator weremixed with raw material rubbers (a natural rubber and a high-cis BR) ina Banbury type internal mixer, and sulfur and the vulcanizingaccelerator were then added to the resultant master batch in an openroll mill to prepare rubber compositions. Tables 4 to 8 show themeasured results of tests, i.e., wear resistance, workability and carbondispersibility of the respective rubber compositions.

Table 9 shows characteristic values of CTAB method surface areas and of24M4DBP absorption values of carbon blacks A to E used in the examplesin Tables 1 to 8. Furthermore, Table 10 shows characteristic values ofthe ML₁₊₄ (100° C.) [Mooney viscosity] of high-cis BRs A to C used inthe examples in Tables 4 to 8.

(2) Tests

(1) Wear resistance

Separately from a tire used to evaluate controllability and stability, atread of a tire was cut into two portions on the periphery of the tire,and for one of the tread, the tire having the tread formed from a treadrubber composition of Comparative Example 1 was prepared for a test.This test tire was run on the surface of a paved road as much as 50,000km, and a depth of wear was then measured. Afterward, mileages requiredto wear a depth of 1 mm were compared, and they were represented withindexes, assuming the mileage of Comparative Example 1 to be 100. It ismeant that the larger the index is, the better the wear resistance is.(2) Workability

The workability was evaluated by a Mooney viscosity. This Mooneyviscosity was measured at 100° C. by the use of a Mooney viscometer madeby Shimadzu Seisakusho Ltd.

The workability test was carried out in accordance with JIS K6300 toobtain an ML₁₊₄ (a Mooney viscosity value after preheating for 1 minuteand then driving for 4 minutes). It is meant that the smaller the indexis, the better the workability is.

(3) Carbon dispersibility

A degree of dispersion was measured in accordance with an ASTM D-2663Bmethod by the use of a microscope. It is meant that the larger the indexis, the better the dispersibility is.

                  TABLE 1                                                         ______________________________________                                                   Ex-   Ex-     Comp.   Ex-   Comp.                                             ample ample   Ex.     ample Ex.                                               1     2       1       3     2                                      ______________________________________                                        Natural rubber                                                                             100.0   100.0   100.0 100.0 100.0                                Carbon black A                                                                             60.0    60.0    60.0  60.0  60.0                                 Stearic acid 2.0     2.0     2.0   2.0   2.0                                  Zinc white   3.0     3.0     3.0   3.0   3.0                                  Sulfur       1.0     1.0     1.0   1.0   1.0                                  Vulcanizing  1.2     1.2     1.2   1.2   1.2                                  accelerator (CZ)                                                              Antioxidant  1.0     1.0     1.0   1.0   1.0                                  DCO-FA*.sup.1                                                                              2.0     4.0     0.0   15.0  --                                   Additive 2*.sup.2                                                                          --      --      --    --    2.0                                  Wear resistance                                                                            120     115     100   110   92                                   (Index)                                                                       Workability (Index)                                                                        95      90      110   72    96                                   Carbon dispersi-                                                                           110     110     100   100   101                                  bility (Index)                                                                ______________________________________                                         *.sup.1 DCO-FA was a dehydrated castor oil fatty acid containing 35% by       weight of a conjugated diene acid and having an iodine value of 156 (the      same shall apply to Tables 1 to 8).                                           *.sup.2 A dehydrated castor oil.                                         

                                      TABLE 2                                     __________________________________________________________________________                Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                     Ex. 3                                                                             Ex. 4                                                                             Ex. 5                                                                             Ex. 6                                                                             Ex. 7                                                                             Ex. 8                                         __________________________________________________________________________    Natural rubber                                                                            100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                         Carbon black A                                                                            60.0                                                                              60.0                                                                              --  --  --  --                                            Carbon black B                                                                            --  --  60.0                                                                              60.0                                                                              --  --                                            Carbon black C                                                                            --  --  --  --  60.0                                                                              60.0                                          Stearic acid                                                                              2.0 2.0 2.0 2.0 2.0 2.0                                           Zinc White  3.0 3.0 3.0 3.0 3.0 3.0                                           Sulfur      1.0 1.0 1.0 1.0 1.0 1.0                                           Vulcanizing 1.2 1.2 1.2 1.2 1.2 1.2                                           accelerator (CZ)                                                              Antioxidant 1.0 1.0 1.0 1.0 1.0 1.0                                           DCO-FA      --  --  2.0 0.0 2.0 0.0                                           Additive 3*.sup.3                                                                         2.0 --  --  --  --  --                                            Additive 4*.sup.4                                                                         --  2.0 --  --  --  --                                            Wear resistance                                                                           98  97  92  90  90  86                                            Workability 100 100 122 156 120 132                                           Carbon dispersibility                                                                     100 100 90  80  90  82                                            __________________________________________________________________________     *.sup.3 Oleic acid.                                                           *.sup.4 A mixture of DCOFA and linoleic acid in a mixing ratio of 1:1.   

                                      TABLE 3                                     __________________________________________________________________________                Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                     Ex. 9                                                                             Ex. 10                                                                            Ex. 11                                                                            Ex. 12                                                                            Ex. 13                                                                            Ex. 14                                        __________________________________________________________________________    Natural rubber                                                                            100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                         Carbon black A                                                                            --  --  --  --  30.0                                                                              100.0                                         Carbon black D                                                                            60.0                                                                              60.0                                                                              --  --  --  --                                            Carbon black E                                                                            --  --  60.0                                                                              60.0                                                                              --  --                                            Stearic acid                                                                              2.0 2.0 2.0 2.0 2.0 2.0                                           Zinc white  3.0 3.0 3.0 3.0 3.0 3.0                                           Sulfur      1.0 1.0 1.0 1.0 1.0 1.0                                           Vulcanizing 1.2 1.2 1.2 1.2 1.2 1.2                                           accelerator (CZ)                                                              Antioxidant 1.0 1.0 1.0 1.0 1.0 1.0                                           DCO-FA      2.0 0.0 2.0 0.0 2.0 0.0                                           Wear resistance                                                                           97  94  87  80  81  88                                            Workability 85  93  75  83  61  166                                           Carbon dispersibility                                                                     102 100 121 110 121 80                                            __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________               Example                                                                            Comp.                                                                             Example                                                                            Comp.                                                                             Example                                                                            Comp.                                                  4    Ex. 15                                                                            5    Ex. 16                                                                            6    Ex. 17                                      __________________________________________________________________________    Natural rubber                                                                           70.0 70.0                                                                              50.0 50.0                                                                              90.0 90.0                                        High-cis BR A                                                                            30.0 30.0                                                                              50.0 50.0                                                                              10.0 10.0                                        Carbon black A                                                                           60.0 60.0                                                                              60.0 60.0                                                                              60.0 60.0                                        Stearic acid                                                                             2.0  2.0 2.0  2.0 2.0  2.0                                         Zinc white 3.0  3.0 3.0  3.0 3.0  3.0                                         Sulfur     1.0  1.0 1.0  1.0 1.0  1.0                                         Vulcanizing                                                                              1.2  1.2 1.2  1.2 1.2  1.2                                         accelerator (CZ)                                                              Antioxidant                                                                              1.0  1.0 1.0  1.0 1.0  1.0                                         DCO-FA     2.0  0.0 2.0  0.0 2.0  0.0                                         Wear resistance                                                                          120  100 130  99  98   85                                          Workability                                                                              90   100 95   130 80   100                                         Carbon dispersibility                                                                    110  90  100  80  110  95                                          __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________               Example                                                                            Comp.                                                                             Example                                                                            Comp.                                                                             Comp.                                                                             Comp.                                                   7    Ex. 18                                                                            8    Ex. 19                                                                            Ex. 20                                                                            Ex. 21                                       __________________________________________________________________________    Natural rubber                                                                           20.0 20.0                                                                              60.0 30.0                                                                              70.0                                                                              70.0                                         High-cis BR A                                                                            80.0 80.0                                                                              30.0 30.0                                                                              --  --                                           High-cis BR B                                                                            --   --  --   --  30.0                                                                              30.0                                         IR2200*.sup.5                                                                            --   --  10.0 40.0                                                                              --  --                                           Carbon black A                                                                           60.0 60.0                                                                              60.0 60.0                                                                              60.0                                                                              60.0                                         Stearic acid                                                                             2.0  2.0 2.0  2.0 2.0 2.0                                          Zinc white 3.0  3.0 3.0  3.0 3.0 3.0                                          Sulfur     1.0  1.0 1.0  1.0 1.0 1.0                                          Vulcanizing                                                                              1.2  1.2 1.2  1.2 1.2 1.2                                          accelerator (CZ)                                                              Antioxidant                                                                              1.0  1.0 1.0  1.0 1.0 1.0                                          DCO-FA     2.0  0.0 2.0  0.0 2.0 0.0                                          Wear resistance                                                                          99   92  115  90  98  95                                           Workability                                                                              140  160 100  100 86  89                                           Carbon dispersibility                                                                    60   50  110  100 106 103                                          __________________________________________________________________________     *.sup.5 Isoprene rubber (trade name of a rubber made by Japan Synthetic       Rubber Co., Ltd.)                                                        

                                      TABLE 6                                     __________________________________________________________________________               Example                                                                            Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                    9    Ex. 22                                                                            Ex. 23                                                                            Ex. 24                                                                            Ex. 25                                                                            Ex. 26                                        __________________________________________________________________________    Natural rubber                                                                           70.0 70.0                                                                              70.0                                                                              70.0                                                                              70.0                                                                              70.0                                          High-cis BR A                                                                            --   --  30.0                                                                              30.0                                                                              30.0                                                                              30.0                                          High-cis BR C                                                                            30.0 30.0                                                                              --  --  --  --                                            Carbon black A                                                                           60.0 60.0                                                                              --  --  --  --                                            Carbon black B                                                                           --   --  60.0                                                                              60.0                                                                              --  --                                            Carbon black C                                                                           --   --  --  --  60.0                                                                              60.0                                          Stearic acid                                                                             2.0  2.0 2.0 2.0 2.0 2.0                                           Zinc white 3.0  3.0 3.0 3.0 3.0 3.0                                           Sulfur     1.0  1.0 1.0 1.0 1.0 1.0                                           Vulcanizing                                                                              1.2  1.2 1.2 1.2 1.2 1.2                                           accelerator (CZ)                                                              Antioxidant                                                                              1.0  1.0 1.0 1.0 1.0 1.0                                           DCO-FA     2.0  0.0 2.0 0.0 2.0 0.0                                           Wear resistance                                                                          104  98  80  80  109 108                                           Workability                                                                              127  156 140 150 137 143                                           Carbon dispersibility                                                                    80   50  50  50  103 103                                           __________________________________________________________________________

                  TABLE 7                                                         ______________________________________                                                   Comp. Comp.   Ex-     Comp. Comp.                                             Ex.   Ex.     ample   Ex.   Ex.                                               27    28      10      29    30                                     ______________________________________                                        Natural rubber                                                                             70.0    70.0    70.0  70.0  70.0                                 High-cis BR A                                                                              30.0    30.0    30.0  30.0  30.0                                 Carbon black A                                                                             30.0    30.0    90.0  90.0  60.0                                 Stearic acid 2.0     2.0     2.0   2.0   2.0                                  Zinc white   3.0     3.0     3.0   3.0   3.0                                  Sulfur       1.0     1.0     1.0   1.0   1.0                                  Vulcanizing  1.2     1.2     1.2   1.2   1.2                                  accelerator (CZ)                                                              Antioxidant  1.0     1.0     1.0   1.0   1.0                                  DCO-FA       2.0     0.0     2.0   0.0   --                                   Additive 2*.sup.2                                                                          --      --      --    --    2.0                                  Wear resistance                                                                            72      72      99    88    95                                   Workability  68      71      152   168   103                                  Carbon dispersibility                                                                      121     120     70    60    95                                   ______________________________________                                         *.sup.2 A dehydrated castor oil.                                         

                                      TABLE 8                                     __________________________________________________________________________                Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                                     Ex. 31                                                                            Ex. 32                                                                            Ex. 33                                                                            Ex. 34                                                                            Ex. 35                                                                            Ex. 36                                        __________________________________________________________________________    Natural rubber                                                                            70.0                                                                              70.0                                                                              70.0                                                                              70.0                                                                              70.0                                                                              70.0                                          High-cis BR A                                                                             30.0                                                                              30.0                                                                              30.0                                                                              30.0                                                                              30.0                                                                              30.0                                          Carbon black A                                                                            30.0                                                                              30.0                                                                              90.0                                                                              90.0                                                                              60.0                                                                              60.0                                          Stearic acid                                                                              2.0 2.0 2.0 2.0 2.0 2.0                                           Zinc white  3.0 3.0 3.0 3.0 3.0 3.0                                           Sulfur      1.0 1.0 1.0 1.0 1.0 1.0                                           Vulcanizing 1.2 1.2 1.2 1.2 1.2 1.2                                           accelerator (CZ)                                                              Antioxidant 1.0 1.0 1.0 1.0 1.0 1.0                                           DCO-FA      --  --  0.05                                                                              22.0                                                                              2.0 0.0                                           Additive 3*.sup.3                                                                         2.0 --  --  --  2.0 --                                            Additive 4*.sup.4                                                                         --  2.0 --  --  --  2.0                                           Aroma oil   --  --  --  --  10.0                                                                              10.0                                          Wear resistance                                                                           95  101 92  94  95  86                                            Workability 104 110 90  100 90  87                                            Carbon dispersibility                                                                     98  100 110 110 98  106                                           __________________________________________________________________________     *.sup.3 Oleic acid.                                                           *.sup.4 A mixture of DCOFA and linoleic acid in a mixing ratio of 1:1.   

                  TABLE 9                                                         ______________________________________                                                 CTAB Method Surface                                                           Area (m.sup.2 /g)                                                                           24M4DBP (ml/g)                                         ______________________________________                                        Carbon black A                                                                           180             125                                                Carbon black B                                                                           230             120                                                Carbon black C                                                                           180             160                                                Carbon black D                                                                           180             80                                                 Carbon black E                                                                           100             80                                                 ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                                 Mw      Mw/Mn     ML.sub.1+4 (100° C.)                        ______________________________________                                        High-cis BR A                                                                            550,000   2.8       43                                             High-cis BR B                                                                            360,000   3.9       30                                             High-cis BR C                                                                            1,000,000 2.9       130                                            ______________________________________                                    

Consideration of Tables 1 to 3

Examples 1 to 3 are concerned with rubber compositions for tire treadsin which carbon blacks and organic unsaturated fatty acids havingcharacteristic values and blend ratios in the claims of the presentinvention are blended with 100 parts by weight of a natural rubber.Thus, it is apparent that these rubber compositions are excellent inwear resistance and have low Mooney viscosity values, and thereforeworkability and carbon dispersibility are good.

On the contrary, Comparative Examples 1 to 14 are concerned with rubbercompositions for tire treads in which carbon blacks and various organicunsaturated fatty acids having changed characteristic values and blendratios are blended with 100 parts by weight of a natural rubber.

That is to say, in Comparative Example 1, the blend ratio of the organicunsaturated fatty acid of the present invention is outside the range ofthe present invention; in Comparative Examples 2 to 4, the organicunsaturated fatty acid of the present invention is not used; inComparative Examples 5 to 12, characteristic values of CTAB methodsurface areas and 24M4DBP absorption values of the carbon blacks of thepresent invention are outside the range of the present invention; and inComparative Examples 13 and 14, amounts of the carbon blacks of thepresent invention are outside the range of the present invention.

It is apparent that in Comparative Examples 1 to 14, the wear resistanceis poor and Mooney viscosity values are high, so that workability andcarbon dispersibility are also poor.

Consideration of Tables 4 to 8

Examples 4 to 10 are concerned with rubber compositions for tire treadsin which carbon blacks and organic unsaturated fatty acids havingcharacteristic values and blend ratios in the claims of the presentinvention are blended with 100 parts by weight of mixed rubberscomprising natural rubbers and high-cis BRs in various mixing ratios. Itis apparent that these rubber compositions are excellent in wearresistance and have low Mooney viscosity values, and thereforeworkability and carbon dispersibility both are good.

On the contrary, Comparative Examples 15 to 36 are concerned with rubbercompositions for tire treads in which carbon blacks and various organicunsaturated fatty acids having changed characteristic values and blendratios are blended with 100 parts by weight comprising mixed rubbers ofnatural rubbers and high-cis BRs in various mixing ratios.

That is to say, in Comparative Examples 15 to 18, the mixing ratios ofthe mixed rubbers comprising the natural rubbers and the high-cis BRsand the blend ratios of the organic unsaturated fatty acids of thepresent invention are outside the range of the present invention; inComparative Example 19, the mixing ratio of the mixed rubber comprisingthe natural rubber and the high-cis BR is outside the range of thepresent invention; in Comparative Examples 20 to 22, characteristicvalues of the high-cis BRs of the present invention are outside therange of the present invention; in Comparative Examples 23 to 26,characteristic values of CTAB method surface areas and 24M4DBPabsorption values of the carbon blacks of the present invention areoutside the range of the present invention; in Comparative Examples 27and 28, the blend ratios of the carbon blacks of the present inventionare outside the range of the present invention; in Comparative Examples29 to 32, the organic unsaturated fatty acid of the present inventionare outside the range of the present invention; in Comparative Examples33 and 34, the blend ratios of the organic unsaturated fatty acids ofthe present invention are outside the range of the present invention;and in Comparative Examples 35 and 36, the organic unsaturated fattyacid of the present invention is mixed with organic unsaturated fattyacids outside the range of the present invention.

In Comparative Examples 15 to 36, it is apparent that the wearresistance is poor, the Mooney viscosity is high, so that workabilityand carbon dispersibility both are bad.

Possibility of Industrial Utilization

As described above, a rubber composition for a tire tread regarding thepresent invention has a low Mooney viscosity, so that workability andcarbon dispersibility both are good. Thus, this rubber composition forthe tire tread can be suitably used for a pneumatic tire for heavy loadhaving an excellent wear resistance.

We claim:
 1. A rubber composition for a tire tread which is obtained byblending 100 parts by weight of a natural rubber and/or a dienesynthetic rubber with40 to 100 parts by weight of a carbon black havingcharacteristic values of a cetyltriammonium bromide adsorption specificsurface area (CTAB) being 90 to 220 m² /g and a 24M4DBP absorption valuebeing 90 to 140 ml/100 g, and 0.1 to 20 parts by weight of an organicunsaturated fatty acid having two or more carbon double bonds in itsmolecule which includes 35% by weight or more of a conjugated diene acidhaving at least one conjugated carbon-carbon double bonds in itsmolecule.
 2. The rubber composition for a tire tread according to claim1 wherein the carbon black has a cetyltriammonium bromide adsorptionspecific surface area (CTAB) of 120 to 220 m² /g and a 24M4DBPabsorption value of 100 to 140 ml/100 g.
 3. The rubber composition for atire tread according to claim 1 wherein the diene synthetic rubber isone selected from the group consisting of a polybutadiene, astyrene•butadiene copolymer rubber, a synthetic polyisoprene rubber, achloroprene rubber, a butyl rubber and an ethylene•propylene terpolymerrubber.
 4. The rubber composition for a tire tread according to claim 1wherein the natural rubber and/or a cis-1,4-polybutadiene rubber iscontained in an amount of 70% by weight or more in 100 parts by weightof the total rubber components.
 5. The rubber composition for a tiretread according to claim 4 wherein the cis-1,4-polybutadiene rubber hascharacteristic values of the following (a):(a) an Mw is 400,000 or moreand an Mw/Mn is less than 4.0 wherein the Mw is a weight-averagemolecular weight and an Mn is a number-average molecular weight.
 6. Therubber composition for a tire tread according to claim 4 or 5 wherein amixing ratio of the natural rubber and the cis-1,4-polybutadiene rubberis such that the amount of the natural rubber is 70% by weight or more.